1. Field of the Invention
The present invention relates to a scanning optical device and, more particularly, to a scanning optical device suitable for image forming apparatuses such as a digital copying machine, a laser beam printer (LBP), and the like, which record image information at a desired scanning line density by appropriately setting imaging magnifications in the main and sub-scanning directions of an optical system (scanning optical system) of the device when the surface to be scanned of a photosensitive body or the like is simultaneously optically scanned using a plurality of light beams.
2. Related Background Art
In recent years, in a scanning optical device used in a digital copying machine or the like, as a method of achieving high-speed output (high-speed printing), for example, the rotation speed of a rotary polygonal mirror used in the scanning optical device is increased. However, such a method has encountered problems of temperature rise and limited rotational speed of the motor arising from high-speed rotation, and limited high-speed output due to high-speed image clocks for modulating a laser device serving as a write means.
On the other hand, as another method of achieving high-speed output, different regions (a plurality of lines) on the surface of a recording medium are simultaneously scanned using a plurality of light beams to simultaneously write image information for these lines.
FIG. 1 is a schematic view showing principal part of an optical system of such scanning optical device using a plurality of light beams, and shows the state in the main scanning section including the main scanning direction.
Referring to FIG. 1, a light source means 31 comprises a monolithic multibeam laser prepared by forming a plurality of light emitting points (laser devices) on a single substrate surface. A collimator lens 32 converts a plurality of light beams emitted by the light source means 31 into collimated light beams. An aperture stop 33 adjusts the beam sizes of beams that pass the stop 33. A cylindrical lens 34 has a predetermined refractive power in only the sub-scanning direction. Note that the collimator lens 32, aperture stop 33, cylindrical lens 34, and the like constitute an optical means (incident optical system) for guiding a plurality of light beams emitted by the light source means 31 to an optical deflector 35 as a deflection means.
The optical deflector 35 comprises a rotary polygonal mirror, which is rotated at a constant speed in the direction of an arrow A by a driving means (not shown). An f-.theta. lens system 36 serves as an imaging optical system, which focuses a plurality of light beams deflected and reflected by the optical deflector 35 and images them at different exposure positions on the surface of a photosensitive body 37 as the surface to be scanned.
In such scanning optical device, a plurality of light beams are optically modulated based on an image signal and are emitted by the light source means 31. These light beams are converted into substantially collimated light beams by the collimator lens 32, and their beam sizes are adjusted by the aperture stop 33. The adjusted light beams then enter the cylindrical lens 34. The cylindrical lens 34 outputs the collimated light beams intact in a main scanning section, but converges and images them as substantially linear images on a deflection surface (reflection surface) 35a of the optical deflector 35 in a sub-scanning section perpendicular to the plane of the drawing of FIG. 1. The light beams deflectively reflected by the optical deflector 35 pass through the imaging optical system 36, and form beam spots on different regions on the surface of the photosensitive body 37, thus sequentially forming (recording) image information on the surface of the photosensitive body 37 as a recording medium.
The light source means 31 in FIG. 1 comprises a monolithic multibeam laser prepared by forming a plurality of light emitting points on a single substrate surface, as described above. FIG. 2 shows the positional relationship between the light emitting points on the substrate surface (light emitting point surface) of the monolithic multibeam laser. In FIG. 2, two light emitting points A and B juxtaposed in the main scanning direction are tilted (rotated) a predetermined angle .theta. in the subscanning direction about an optical axis M as the center.
FIG. 2 shows an example of the monolithic multibeam laser 31 having the two light emitting points A and B, which have a light emitting interval L falling within the range from several 10 .mu.m to several 100 .mu.m. When the light emitting interval L between the two light emitting points A and B becomes extremely small, electrical crosstalk is produced between the two light emitting points A and B. Hence, it is a common practice to assure a light emitting interval L of about 100 .mu.m or more.
In the scanning optical device, in order to simultaneously scan a plurality of lines on the surface 37 to be scanned using such monolithic multibeam laser 31, a line interval R.sub.F (.beta..sub.F.times.L.sub.F) on the surface to be scanned is obtained by multiplying an imaging magnification .beta..sub.F of the scanning optical system in the subscanning direction by the interval L.sub.F between the two light emitting points A and B in the subscanning direction. For example, if the two light emitting points A and B are linearly aligned in the subscanning direction, the line interval R.sub.F on the surface 37 to be scanned becomes as large as about 100 .mu.m if the imaging magnification .beta..sub.F in the subscanning direction is equal to or larger than equal magnification. For this reason, the scanning line density (resolution) in the subscanning direction cannot be set at about 400 or 600 dpi.
In order to solve the above-mentioned problem, as shown in FIG. 2, the scanning line density (resolution) in the subscanning direction is increased by arranging the two light emitting points A and B juxtaposed in the main scanning direction while being tilted by the predetermined angle .theta. about the optical axis M as the center and narrowing an apparent line interval R.sub.F in the subscanning direction by setting: EQU L.sub.F =L.times.sin .theta.
where L is the light emitting interval between the two light emitting points A and B on the substrate surface and L.sub.F is the interval between these points A and B in the subscanning direction.
However, the conventional scanning optical device does not give any consideration to the interval between the two light emitting points A and B in the main scanning direction, and the imaging magnification of the scanning optical system in the main scanning direction.
Normally, the light emitting timings for emitting a plurality of light beams in the main scanning direction can only be discretely selected. For this reason, if large errors are produced between the actual beam interval of the plurality of light beams in the main scanning direction on the surface to be scanned and the beam interval defined by the discrete light emitting timings, positional deviations of the image are generated in the main scanning direction.